Enhanced magnetic performance of bulk nanocrystalline MnAl–C prepared by high pressure compaction of gas atomized powders

High density MnAl–C magnets with enhanced coercivity and remanent magnetization were prepared by high-pressure compaction of the \(\uptau \)-phase obtained by annealing the as-prepared gas-atomized powders, which are spherical in shape with size in the range of 1–7 \(\upmu \)m. The as-prepared gas-atomized powders were composed of \(\upvarepsilon \)- as the major phase and \(\upgamma _{2}\)- as the minor phase. The massive phase transformation of \(\upvarepsilon \rightarrow \uptau \) in the gas-atomized powders occurs at 720 K and accomplishes at 806 K, both of which are lower than those of the water-quenched \(\upvarepsilon \)-MnAl–C alloys with the same composition. An optimized temperature of 760 K, at which the decomposition of metastable \(\uptau \)-phase was minimized, was selected to prepare the ferromagnetic \(\uptau \)- from the \(\upvarepsilon \)-phase. The spherical \(\uptau \)-phase powders were pressed at room temperature into two dimensional plates that stack along the direction of compaction, forming high density (98.6%) bulk magnets that exhibit larger coercivity and higher remanent magnetization than that of the \(\uptau \)-phase powders. The grain size of the compacted samples was observed to be in the range of 10–100 nm. The coercivity (0.34 T) of the dense samples is twice as large as that of the \(\uptau \)-phase powders, owing to the refined grain size and enlarged dislocation density resulting from high-pressure compaction.